Adsorption processes are widely used in industry for separation and purification of fluid mixtures. The separation is based on preferential adsorption of adsorbable components on the active surface of solid adsorbent particles. For efficient adsorption, the adsorbent must have large active surface areas with high adsorptive capacity. The commonly used adsorbents, such as molecular sieve zeolites, activated carbon, alumina and silica gel, have adsorptive areas of at least 200 m.sup.2 /g. Most of these areas are located at the interior of the porous adsorbent particles.
Most industrial adsorption processes are carried out in a fixed-bed adsorber, wherein the adsorbent granules are packed and immobilized in a cylindrical vessel. As a fluid mixture to be separated or purified is passed through the adsorbent packing via the void spaces among the granules, the adsorbable components in the mixture are taken up and retained by the adsorbent. Apart from the adsorptive capacity of the adsorbent material, adsorption rate and pressure drop are two important factors that must be considered in adsorber design.
Pressure drop through the adsorber column should be minimized to reduce fluid pumping and compression costs. In addition, high pressure drop can cause movement or fluidization of the adsorbent particles, resulting in serious attrition and loss of the adsorbent material.
Adsorption rate has a significant bearing on the efficiency of the adsorption process. The rate is usually determined by the mass transfer resistance between the bulk fluid phase and the internal adsorption surfaces of the adsorbent particles. Slow adsorption rate due to large mass transfer resistance will result in a long mass transfer zone (MTZ) migrating through the adsorber column in the direction of the fluid stream during the course of the adsorption operation. The adsorbent in the region upstream of the MTZ is substantially saturated with adsorbate, while that downstream of the MTZ is essentially free of adsorbate. Within the MTZ, the adsorbent is only partially saturated with adsorbate. The adsorption step must be terminated just before the leading edge of the MTZ reaches the adsorber outlet in order to avoid the breakthrough of adsorbate in the effluent stream. A long MTZ which contains a large quantity of the partially utilized adsorbent will, therefore, result in inefficient use of the adsorption capacity.
Both the pressure drop and the mass transfer resistance are strongly influenced by the size of the adsorbent particles. Changing the particle size, unfortunately, has opposite effects on these two important design factors. This elaborated below:
a) The size of the interstitial void spaces among the adsorbent particles in a fixed-bed packing is proportional to the size of the particles. Since the resistance to the fluid flow through the adsorbent packing is inversely proportional to the size of these void spaces, the use of small adsorbent particles will cause high pressure drop. For this reason, the sizes of particles of commercial adsorbents for fixed-bed adsorber operation are generally larger than 2.about.3 mm in diameter. Adsorbents of smaller particle sizes, such as zeolite crystals (less than 10 micrometer), are pelletized using binding material to suitable sizes.
b) Almost all the surface areas of commercial adsorbents are located at the interior of the adsorbent particle. For adsorption to occur, the adsorbate needs to be transported from the external fluid stream to the interior surface of the particle. The transport rate is generally dominated by two mass transfer mechanisms in series: (a) interfacial mass transfer--diffusion through the fluid boundary layer surrounding the external surface of the adsorbent particle; and (b) intraparticle mass transfer--diffusion through the internal pore space (micropores and macropores) of the particle to its interior surface where adsorption takes place. The size of the particle has significant effects on the rates of these two diffusion processes. Small particles not only offer large fluid/solid contact areas for interfacial mass transfer, but also reduce the path length for intraparticle diffusion. Hence, small adsorbent particles will increase adsorption rate and result in a narrow MTZ for efficient adsorption operation.
The above analysis indicates that small adsorbent particles are desirable for efficient adsorption, but the minimum particle size is limited by acceptable hydrodynamic operating conditions of the fixed-bed adsorber. Such a concept also applies to other processes requiring effective contact and interaction between a fluid stream and solid particles, such as a heterogeneous catalytic reaction process involving an adsorption step in the reaction mechanism. The use of small catalyst particles will enhance mass transfer between the catalyst and surrounding fluid carrying the reactants, but it will also increase pressure drop through the reactor bed.
It would, therefore, be desirable to provide a fixed-bed module containing adsorbent or catalyst characterized by a relatively small particle size and yet still able to operate with an acceptable pressure drop.
In a conventional fixed-bed module using small solid particles, in order to reduce pressure drop it is necessary to employ a thin-bed module with large cross sectional area to reduce the flow path length and velocity. This, however, will require a costly large-diameter pressure vessel to house the particles.
Two unconventional fixed-bed modules described in U.S. Pat. Nos. 5,139,668 (Pan and McMinis) and 5,338,450 (Maurer) can use very small solid particles without giving rise to high pressure drop. In Pan and McMinis, porous hollow fibers are used to immobilize minute solid particles inside the fiber lumina. The void spaces between the fibers provide an unobstructed passageway for fluid flow with low pressure drop. But such a module requires expensive and delicate hollow fibers to construct.
In Maurer, a single layer of adsorbent material is spirally wound into a cylindrical adsorber module, with adjacent spiral loops being separated by two layers of flow channels. The resulting adsorber can use minute adsorbent particles without high pressure drop, because it has large area of porous channel walls to distribute a fluid stream through a relatively thin layer of adsorbent packing. There are, however, a couple of disadvantages associated with the Maurer spiral-wound adsorber module. The available space within the module that can be packed with adsorbent is typically less than 50%, because the number of the flow channel loops is twice that of the adsorbent loops. These channels can easily occupy more than 50% of the total available space within the module (assuming that the thickness of the flow channel is half of that of the adsorbent layer as specified in the Maurer examples). Thus the adsorber has poor utilization of physical space for adsorbent packing, and requires a much larger housing vessel than the conventional column-type adsorber to accommodate a given adsorption capacity. A further disadvantage of the Maurer adsorber is that it does not have structural integrity necessary for high pressure operation, such as pressure swing adsorption. This is because the periphery of the spiral-wound structure is not totally supported by the housing vessel. Any internal pressure within the flow channel will force the spiral structure to unfold and disintegrate. Hence, the adsorber can only be used for the thermal swing adsorption process (no pressure swing) as specified by Maurer.
Accordingly, the objects and advantages of the present invention are:
a) to provide a spiral fixed-bed module that can function as the conventional thin-bed module for effective fluid/particles contact and interaction, but without the need for a large-diameter pressure vessel;
b) to provide a spiral fixed-bed module with high particle packing capacity; and
c) to provide a spiral fixed-bed module with high pressure operation capability.